endosymbiont - (n.)

A cell, most often of prokaryotic type, adopted as a stable component of a eukaryotic cell : Several cell organelles, including chloroplasts, mitochondria, and, perhaps, peroxisomes and hydrogenosomes, are derived from endosymbiotic bacteria. ~ See Also: Chloroplast, Hydrogenosome, Mitochondrion, Peroxisome.